The invention relates to a write-once optical data carrier comprising at least one phthalocyanine dye as light-absorbent compound in the information layer, to a process for its production, and to the application of the above-mentioned dyes to a polymer substrate, particularly polycarbonate, by spin coating, vapor deposition, or sputtering.
Write-once optical data carriers using specific light-absorbent substances or mixtures thereof are particularly suitable for use in high-density writeable optical data stores which operate with blue laser diodes, in particular GaN or SHG laser diodes (360 to 460 nm) and/or for use in DVD-R or CD-R disks that operate with red (635 to 660 nm) or infrared (780 to 830 nm) laser diodes.
The write-once compact disk (CD-R, 780 nm) has recently experienced enormous volume growth and represents the technically established system.
The next generation of optical data stores—DVDs—has recently been introduced onto the market. Through the use of shorter-wavelength laser radiation (635 to 660 nm) and higher numerical aperture (NA), the storage density can be increased. The write-once format in this case is DVD-R.
Today, optical data storage formats which use blue laser diodes (based on GaN, JP-A 08-191,171 or Second Harmonic Generation SHG JP-A 09-50,629) (360 nm to 460 nm) with high laser power are being developed. Writeable optical data stores will therefore also be used in this generation. The achievable storage density depends on the focusing of the laser spot in the information plane. Spot-size scales with the laser wavelength λ/NA. NA is the numerical aperture of the objective lens used. In order to obtain the highest possible storage density, the use of the smallest possible wavelength λ is the aim. At present 390 nm is possible on the basis of semiconductor laser diodes.
The patent literature describes dye-based writeable optical data stores which are equally suitable for CD-R and DVD-R systems (JP-A 11-43,481 and JP-A 10-181,206). To achieve a high reflectivity and a high modulation height of the read-out signal and also to achieve sufficient sensitivity in writing, use is made of the fact that the IR wavelength 780 nm of CD-Rs is located at the foot of the long wavelength flank of the absorption peak of the dye and the red wavelength of 635 nm or 650 nm of DVD-Rs is located at the foot of the short wavelength flank of the absorption peak of the dye (cf. EP-A 519,395 and WO-A 00/09522). In JP-A 02-557,335, JP-A 10-58,828, JP-A 06-336,086, JP-A 02-865,955, WO-A 09 917,284, and U.S. Pat. No. 5,266,699, this concept is extended to the 450 nm working wavelength region on the short wavelength flank and the red and IR region on the long wavelength flank of the absorption peak.
Apart from the above-mentioned optical properties, the writeable information layer comprising light-absorbent organic substances must have a substantially amorphous morphology to keep the noise signal during writing or reading as small as possible. For this reason, it is particularly preferred that crystallization of the light-absorbent substances be prevented in the application of the substances by spin coating from a solution, by sputtering, or by vapor deposition and/or sublimation during subsequent covering with metallic or dielectric layers under reduced pressure.
The amorphous layer comprising light-absorbent substances preferably has a high heat distortion resistance, since otherwise further layers of organic or inorganic material which are applied to the light-absorbent information layer by sputtering or vapor deposition would form blurred boundaries due to diffusion and thus adversely affect the reflectivity. Furthermore, a light-absorbent substance that has insufficient heat distortion resistance can, at the boundary to a polymeric support, diffuse into the latter and once again adversely affect the reflectivity.
A light-absorbent substance for which the vapor pressure is too high can sublime during the above-mentioned deposition of further layers in a high vacuum and thus reduce the layer thickness to below the desired value. This once again has an adverse effect on the reflectivity.
It is therefore an object of the invention to provide suitable compounds that satisfy the high requirements (e.g., light stability, favourable signal/noise ratio, damage-free application to the substrate, and the like) for use in the information layer in a write-once optical data carrier, particularly for high-density writeable optical data store formats in a laser wavelength range from 360 to 460 nm.
Surprisingly, it has been found that specific light-absorbent phthalocyanine compounds can satisfy the above-mentioned requirement profile particularly well. Phthalocyanines display an intense absorption in wavelength range 360 to 460 nm which is important for the laser, viz. the B or Soret bands.